1. Field of the Invention
The present invention relates to an electron microscope and, more particularly, to a transmission electron microscope equipped with an energy filter.
2. Description of Related Art
Conventionally, transmission electron microscopes (TEMs) equipped with energy filters have been offered. A transmission electron microscope equipped with an energy filter can select and image only electron rays having certain energies from electrons transmitted through a specimen. The microscope has various functions: (1) it can enhance the contrast and resolution of the image; (2) it can obtain an energy-loss spectrum; and (3) it can obtain a two-dimensional distribution of constituent elements of a specimen.
FIG. 5 is a diagram showing the configuration ranging from a specimen to the final image plane of a transmission electron microscope equipped with an energy filter. An electron beam transmitted through a specimen 3a is focused by an objective lens 4 and forms an image. Then, the beam enters an intermediate lens system 5 consisting of a first intermediate lens 5a, a second intermediate lens 5b, and a third intermediate lens 5c. The intermediate lens system 5 brings the image created by the objective lens 4 into focus at an entrance window plane 24 of the energy filter 6. That is, the lenses located ahead of the energy filter 6 focus a crossover image and a microscope image in the entrance window plane 24 and entrance image plane 25, respectively, of the energy filter 6.
The energy filter 6 has a function of selecting only electrons having certain energies out of electrons transmitted through the specimen 3a. The operation of the energy filter 6 is described briefly below. Electrons are directed at the specimen 3a by a condenser lens 2 and pass through the specimen 3a. During this process, the electrons interact with atoms and electrons within the specimen 3a. As a result, the electrons lose energy. This is known as energy loss.
A slit 7 is positioned in an exit window plane 27 of the energy filter 6. Only electrons having undergone a certain energy loss are selected by the slit 7. Consequently, the composition or bonding of elements can be known by causing the energy filter 6 to analyze the distribution of electron energy losses.
Furthermore, an image of two-dimensional distribution of energy losses in the specimen 3a can be obtained by creating an image from the electron rays. This can be applied to analysis of the distribution of elements constituting the specimen 3a, or the contrast of the image can be improved.
When the crossover image and microscope image are made to enter the entrance window plane 24 and entrance image plane 25, respectively, the energy filter 6 creates a microscope image and a crossover image in the exit image plane 26 and exit window plane 27, respectively, in this way. Electron rays having certain energies and selected by the energy filter 6 impinge on a projector lens system 8 consisting of a first projector lens 8a and a second projector lens 8b. 
The projector lens system 8 magnifies the microscope image created by the energy filter 6. That is, the lenses located behind the energy filter 6 project the image created by the energy filter 6 onto the final image plane 9a at a desired magnification factor. A fluorescent screen, photographic film, or scintillator of a CCD camera is positioned in the final image plane 9a. An observer directly observes the image or the image is taken by the CCD camera.
The lenses located ahead of the energy filter 6 are made up of an objective lens 4 and an intermediate lens system 5 in the same way as in an ordinary electron microscope equipped with no energy filter. The objective lens 4 acts to magnify an image of a specimen first. The lens magnifies the image at a large magnification factor of about 30 to 100 times. The image is magnified by the intermediate lens system 5, the energy filter 6, and the projector lens system 8 after that. As a result, the image is lastly magnified at a large magnification factor of about 1,000 to 1,000,000 times. The magnification of the projection lens system 8 is about 50 to 200 times. The intermediate lens system 5 acts to magnify and reduce the image. In the lenses of the microscope, the divergence angle is greatest at this objective lens 4. Therefore, the resolution of the microscope depends much on the performance of the objective lens 4. Generally, the performance is improved by increasing the intensity of the magnetic field produced by the objective lens 4 and concentrating the magnetic field distribution is a narrow area. This shortens the focal distance, which, in turn, increases the magnification.
This improves the performance of the electron microscope at high magnifications, but it becomes difficult to achieve low magnifications of about 500 to 10,000 times for the following reason. The image greatly magnified by the objective lens 4 must be demagnified and brought to a focus by the next intermediate lens system 5. For this purpose, it is necessary to focus electrons passing off the optical axis of the lens.
Lenses used in electron microscopes are electromagnetic lenses. It has been shown that these lenses achieve only convex lenses. For this reason, aberration correction relying on a combination with concave lenses as in an optical microscope cannot be made. In recent years, aberration correction systems using multipolar lenses have been often incorporated within electron microscopes. Although such a correction system is very complex and large in size, it is only capable of correcting spherical aberration in the objective lens 4. It is entirely unrealistic to correct off-axis aberrations in the intermediate lens system 5.
Therefore, in electron microscopes where the objective lens 4 has a high magnification, it is customary to restrict the imaging magnification to about 5,000 times or higher by strongly exciting the objective lens 4. In some cases, lower magnifications are achieved using objective lens 4 of lower magnification. In these cases, the performance of the objective lens 4 is low, and applications are limited to electron microscopes with low resolutions.
It is customary to lower the magnification by exciting the objective lens 4 less strongly or turning it off during low-magnification imaging. At this time, however, the performance of the objective lens 4 deteriorates severely, and the resolution of the electron microscope decreases. This implementation is limited to ultra-low magnifications of about less that 1,000 times.
The range of 500 to 10,000 times is a bridge area between optical microscopy and electron microscopy and, hence, is an important range of magnifications. If this range of magnifications is not achieved, the use of the electron microscope will be hindered. Therefore, the objective lens 4 is weakly excited or turned off consciously of the fact that the resolution is insufficient. The magnification is increased with the intermediate lens system 5. Alternative, the magnification is lowered with the intermediate lens system 5 in use while the objective lens 4 is kept excited strongly at the sacrifice of the image quality of peripheral portions.
In an attempt to improve this situation, a technique for achieving low magnifications of less than 10,000 times while maintaining high resolution of the objective lens 4 and good image quality of peripheral portions is disclosed in Japanese Patent Laid-Open No. 61-49363. In particular, an objective minilens for assisting the objective lens 4 is placed close to the specimen surface. The minilens is used together with the objective lens.
In an electron microscope equipped with an energy filter, however, it has been difficult to achieve a low-magnification mode while the objective lens 4 is kept strongly excited.